A NEW REPUBLICAN Donald J. Trump before addressing the American Israel Public Affairs Committee. His worldview does not fit into his party’s recent history.Credit
Doug Mills/The New York Times

Donald J. Trump, the Republican presidential front-runner, said that if elected, he might halt purchases of oil from Saudi Arabia and other Arab allies unless they commit ground troops to the fight against the Islamic State or “substantially reimburse” the United States for combating the militant group, which threatens their stability.

“If Saudi Arabia was without the cloak of American protection,” Mr. Trump said during a 100-minute interview on foreign policy, spread over two phone calls on Friday, “I don’t think it would be around.”

He also said he would be open to allowing Japan and South Korea to build their own nuclear arsenals rather than depend on the American nuclear umbrella for their protection against North Korea and China. If the United States “keeps on its path, its current path of weakness, they’re going to want to have that anyway, with or without me discussing it,” Mr. Trump said.

And he said he would be willing to withdraw United States forces from both Japan and South Korea if they did not substantially increase their contributions to the costs of housing and feeding those troops. “Not happily, but the answer is yes,” he said.

In Mr. Trump’s worldview, the United States has become a diluted power, and the main mechanism by which he would re-establish its central role in the world is economic bargaining. He approached almost every current international conflict through the prism of a negotiation, even when he was imprecise about the strategic goals he sought. He again faulted the Obama administration’s handling of the negotiations with Iran last year — “It would have been so much better if they had walked away a few times,” he said — but offered only one new idea about how he would change its content: Ban Iran’s trade with North Korea.

Mr. Trump struck similar themes when he discussed the future of NATO, which he called “unfair, economically, to us,” and said he was open to an alternative organization focused on counterterrorism. He argued that the best way to halt China’s placement of military airfields and antiaircraft batteries on reclaimed islands in the South China Sea was to threaten its access to American markets.

“We have tremendous economic power over China,” he argued. “And that’s the power of trade.” He did not mention Beijing’s ability for economic retaliation.

Mr. Trump’s views, as he explained them, fit nowhere into the recent history of the Republican Party: He is not in the internationalist camp of President George Bush, nor does he favor President George W. Bush’s call to make it the United States’ mission to spread democracy around the world. He agreed with a suggestion that his ideas might be summed up as “America First.”

“Not isolationist, but I am America First,” he said. “I like the expression.” He said he was willing to reconsider traditional American alliances if partners were not willing to pay, in cash or troop commitments, for the presence of American forces around the world. “We will not be ripped off anymore,” he said.

In the past week, the bombings in Brussels and an accelerated war against the Islamic State have shifted the focus of the campaign trail conversation back to questions of how the candidates would defend the United States and what kind of diplomacy they would pursue around the world.

Mr. Trump explained his thoughts in concrete and easily digestible terms, but they appeared to reflect little consideration for potential consequences. Much the same way he treats political rivals and interviewers, he personalized how he would engage foreign nations, suggesting his approach would depend partly on “how friendly they’ve been toward us,” not just on national interests or alliances.

At no point did he express any belief that American forces deployed on military bases around the world were by themselves valuable to the United States, though Republican and Democratic administrations have for decades argued that they are essential to deterring military adventurism, protecting commerce and gathering intelligence.

Like Richard M. Nixon, Mr. Trump emphasized the importance of “unpredictability” for an American president, arguing that the country’s traditions of democracy and openness had made its actions too easy for adversaries and allies alike to foresee.

“I wouldn’t want them to know what my real thinking is,” he said of how far he was willing to take the confrontation over the islands in the South China Sea, which are remote and lightly inhabited but extend China’s control over a major maritime thoroughfare. But, he added, “I would use trade, absolutely, as a bargaining chip.”

Asked when he thought American power had been at its peak, Mr. Trump reached back 116 years to the turn of the 20th century, the era of another unconventional Republican, Theodore Roosevelt, who ended up leaving the party. His favorite figures in American history, he said, include two generals, Douglas MacArthur and George S. Patton — though he said that, unlike MacArthur, he would not advocate using nuclear weapons except as a last resort. (He suggested MacArthur had pressed during the Korean War to use them against China as a means “to negotiate,” adding, “He played the nuclear card, but he didn’t use it.”)

Mr. Trump denied that he had had trouble finding top members of the foreign policy establishment to advise him. “Many of them are tied up with contracts working for various networks,” he said, like Fox or CNN.

He named three advisers in addition to five he announced earlier in the week: retired Maj. Gen. Gary L. Harrell, Maj. Gen. Bert K. Mizusawa and retired Rear Adm. Charles R. Kubic. They reflected a continuing bias toward former military officers, rather than diplomats or academics with foreign policy experience. General Harrell, a Special Forces veteran, was a commander in the failed “Black Hawk Down” mission in Somalia in 1993. Admiral Kubic, now president of an engineering firm, has been a sharp critic of President Obama’s handling of the attack on Libya that helped oust Col. Muammar el-Qaddafi.

Asked about the briefings he receives and books he has read on foreign policy, Mr. Trump said his main information source was newspapers, “including yours.”

Until recently, his foreign policy pronouncements have largely come through slogans: “Take the oil,” “Build a wall” and ban Muslim immigrants and visitors, at least temporarily. But as he pulls closer to the nomination, he has been called on to elaborate.

Pressed about his call to “take the oil” controlled by the Islamic State in the Middle East, Mr. Trump acknowledged that this would require deploying ground troops, something he does not favor. “We should’ve taken it, and we would’ve had it,” he said, referring to the years in which the United States occupied Iraq. “Now we have to destroy the oil.”

He did not rule out spying on American allies, including leaders like Angela Merkel, the German chancellor, whose cellphone was apparently a target of the National Security Agency. Mr. Obama said the agency would no longer target her phone but made no such commitments about the rest of Germany, or Europe.

“I’m not sure that I would want to be talking about that,” Mr. Trump said. “You understand what I mean by that.”

Mr. Trump was not impressed with Ms. Merkel’s handling of the migrant crisis, however: “Germany is being destroyed by Merkel’s naïveté, or worse,” he said. He suggested that Germany and the Gulf nations should pay for the “safe zones” he wants to set up in Syria for refugees, and for protecting them once built.

An error has occurred. Please try again later.

You are already subscribed to this email.

Throughout the two conversations, Mr. Trump painted a bleak picture of the United States as a diminished force in the world, an opinion he has held since the late 1980s, when he placed ads in The New York Times and other newspapers calling for Japan and Saudi Arabia to spend more money on their own defense.

Mr. Trump’s new threat to cut off oil purchases from the Saudis was part of a broader complaint about the United States’ Arab allies, which many in the Obama administration share: that they often look to the United States to police the Middle East, without putting their own troops at risk. “We defend everybody,” he said. “When in doubt, come to the United States. We’ll defend you. In some cases free of charge.”

But his rationale for abandoning the region was that “the reason we’re in the Middle East is for oil, and all of a sudden we’re finding out that there’s less reason to be there now.” He made no mention of the risks of withdrawal — that it would encourage Iran to dominate the Gulf, that the presence of American troops is part of Israel’s defense, and that American air and naval bases in the region are key collection points for intelligence and bases for drones and Special Operations forces.

Mr. Trump seemed less comfortable on some topics than others. He called the United States “obsolete” in terms of cyberweaponry, although the nation’s capabilities are generally considered on the cutting edge.

In the morning interview, asked if he would seek a two-state or a one-state solution in a peace accord between the Israelis and the Palestinians, he said: “I’m not saying anything. What I’m going to do is, you know, I specifically don’t want to address the issue because I would love to see if a deal could be made.”

But in the evening, saying he had been rushed earlier, he went back to a position outlined Monday to the American Israel Public Affairs Committee, the pro-Israel group. “Basically, I support a two-state solution on Israel,” he said. “But the Palestinian Authority has to recognize Israel’s right to exist as a Jewish state.”

In discussing nuclear weapons — which he said he had learned about from an uncle, John G. Trump, who was on the M.I.T. faculty — he seemed fixated on the large stockpiles amassed in the Cold War. While he referred briefly to North Korean and Pakistani arsenals, he said nothing about a danger that is a cause of great consternation among global leaders: small nuclear weapons that could be fashioned by terrorists.

In criticizing the Iran nuclear deal, he expressed particular outrage at how the roughly $150 billion released to Iran (by his estimate; the number is in dispute) was being spent. “Did you notice they’re buying from everybody but the United States?” he said.

Told that sanctions under United States law still bar most American companies from doing business with Iran, he said: “So, how stupid is that? We give them the money and we now say, ‘Go buy Airbus instead of Boeing,’ right?”

But Mr. Trump, who has been pushed to demonstrate a basic command of international affairs, insisted that voters should not doubt his foreign policy fluency. “I do know my subject,” he said.

A version of this article appears in print on March 27, 2016, on Page A1 of the New York edition with the headline: Foreign Policy, in Trump’s View, Is About Deals. Order Reprints|Today's Paper|Subscribe